Deltorphins (DEL A,B, or C), highly selective opioid heptapetides, were used as molecular and chemical probes for brain opioid receptors. Receptor binding analyses and pharmacological assays using peripheral tissues enabled a delineation of specific molecular properties for agonist/antangonist interactions. Key molecular features of DEL were based on the study of over 180 synthetic analogues: (1) D-Try1,an N-extended TRY1 or a bridged [TRY1-Asp4] in DEL C are unacceptable for binding. (2) A residue with a hydrophobic side-chain is required at position 5, although binding is affected by the sequence in the C-terminal tetrapeptide; inversion of sequence brought about a reversal in selectivity (- ). (3) Substitution by halogens, amino or nitro groups at the para-position of Phe3 modified brain receptor selectivity and bioactivity; whereas [p-BrPhe3] exhibited twice receptor selectivity as DEL C, [p-ClPhe3] and [p-BrPhe3] were nearly 4- and 2-fold higher than DEL C by bioassay (MVD), however, the only correlation between bioactive potency and receptor binding occured with recptors. (4) Modification of the electronic, hydrophobic and conformational properties of Phe3 in DEL C occurred through use of 5 and 6 membered rings and bicyclic structures. In general, these substitutions were deleterious for affinity and selectivity, however, Aic3 and Atc(R,S)3 enhanced affinity; the latter increased selectivity as well. (5) [D-Asp4]-, [Pro4]- or [Abu4]DEL C slighty affected affinities: the former two analogues defined a two-site binding model, while DEL C and [Abu4] DEL C were one-site models. [NaMe Phe 3]- and [Atc(S)3] DEL C revealed that the mimal energy conformer appeared to predict experimentally derived affinities. Futhermore, molecular dynamics calculations implied that the other low energy configurations could describe their conformation during receptor interactions; pseudo-cyclization involving the residues at positions 3 and 4 or replacement by the bicyclic ring structure Atc3 may represent the most desirable solution conformer.